Light emitting diode

ABSTRACT

The present invention relates to a light-emitting diode (LED), which comprises electrodes having a single metal reflective layer. The single metal reflective layer is thicker than the active layer of the LED. Thereby, at least a portion of light emitted from the active layer is reflected by the single metal reflective layer, and thus enhancing the light-emitting efficiency of the LED.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/641,565, filed on Mar. 9, 2015, now U.S. Pat.No. 9,502,616, which claims the right of priority based on TWapplication Serial No. 103132866, filed on Sep. 23, 2014, and thecontent of which are hereby incorporated by reference in theirentireties.

FIELD OF THE INVENTION

The present invention relates generally to a light-emitting diode (LED),and particularly to an LED comprising electrodes having a single metalreflective layer.

BACKGROUND OF THE INVENTION

LEDs are solid-state light-emitting devices formed from semiconductormaterials that comprise, in generally, III-V compound elements such asgallium phosphide, gallium arsenide, or gallium nitride. By biasing thecompound semiconductor, massive electrons and holes encounter andrecombine in an active layer of LED, and electrons will fall to lowerenergy levels with the emission of photons. Therefore, the electricalenergy is converted to the light to achieve the effect of lightemission.

In the traditional LED structure, electrodes might absorb the lightemitted from the active layer, such that the expected light-emittingperformance of LED is reduced. Moreover, the light absorbed by theelectrodes will be converted into heat, resulting in gradual increasetemperature in the electrodes or overheating condition. Thereby,improvement is required.

In order to reduce the absorption of light emitted from the activelayer, one of prior arts is to form a reflection layer under electrodesof an LED. Thereby, the absorption is avoided when the light emittedfrom the active layer contact the electrodes. In addition, because theside surface of the active layer may emit the light, the reflectivelayer may have a special shape such as ladder- or L-shaped in across-section view, so that the light can be multi reflected via thereflection layer and thus enhancing the light-emitting efficiency ofLED.

However, the prior art, which is required to add reflection layers underelectrodes of LED, increases complexity in fabrication process of LED.Besides, according to different light-receiving conditions under thepositive and negative electrode of LED, the reflection layers shall havedifferent structures respectively, that also increase complexity in thefabrication process.

SUMMARY

The present invention discloses a light emitting diode (LED), whichcomprises a stacked semiconductor layer structure, a first electrode,and a second electrode. The stacked semiconductor layer structurecomprises a first-type semiconductor layer, an active layer, and asecond-type semiconductor layer. The active layer is disposed betweenthe first-type and the second-type semiconductor layers. The first andsecond electrodes are disposed on the same side of the stackedsemiconductor layer structure. In addition, the first electrode isdisposed on the first-type semiconductor layer; the second electrode isdisposed on the second-type semiconductor layer. The first electrodeincludes a single metal reflective layer and a pad layer. The pad layeris disposed on the single metal reflective layer. The thickness of thesingle metal reflective layer is greater than that of the active layer.Moreover, the height of the bottom of the single metal reflective layeris lower than the height of the bottom of the active layer and theheight of the top of the single metal reflective layer is higher thanthe height of the top of the active layer. Thereby, at least a portionof the light emitted from the active layer is reflected by the singlemetal reflective layer.

According to an embodiment of the present invention, the LED may furthercomprise a transparent conductive layer disposed between the second-typesemiconductor layer and the second electrode.

According to an embodiment of the present invention, the LED may furthercomprise a transparent conductive layer disposed between the second-typesemiconductor layer and the second electrode. In addition, the height ofthe top of the single metal reflective layer of the first electrode isnot lower than the height of the transparent conductive layer.

According to an embodiment of the present invention, a gap rangedbetween 7-8 um is further located between the first electrode and theactive layer of the LED.

According to an embodiment of the present invention, the material of thefirst electrode of the LED may include one selected from the groupconsisting of chrome, aluminum, platinum, gold, titanium, tantalum,ruthenium, rhodium, silver, nickel, lead, and copper.

According to an embodiment of the present invention, the material of thesingle metal reflective layer of the first electrode of the LED mayinclude one selected from the group consisting of aluminum, platinum,titanium, tantalum, ruthenium, rhodium, silver, nickel, and lead.

According to an embodiment of the present invention, the thickness ofthe single metal reflective layer of the first electrode of the LED isnot less than 1 um.

According to an embodiment of the present invention, the material of thepad layer of the first electrode of the LED includes gold.

According to an embodiment of the present invention, the thickness ofthe pad layer of the first electrode of the LED is not less than 0.8 um.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of the structure according to apreferred embodiment of the present invention;

FIG. 2 shows a three-dimensional view of the structure according to apreferred embodiment of the present invention;

FIG. 3 shows a cross-sectional view of the structure according toanother preferred embodiment of the present invention;

FIG. 4 shows a schematic diagram of the path of light reflectionaccording to a preferred embodiment of the present invention;

FIG. 5A shows the comparison in electrical property of experimentalresults between a regular LED and the LED having a single metalreflective layer with different thicknesses according to the presentinvention;

FIG. 5B shows the comparison in luminous intensity of experimentalresults between a regular LED and the LED having a single metalreflective layer with different thicknesses according to the presentinvention; and

FIGS. 6A-6D show comparisons in the electrical property of experimentalresults between a regular LED and the LED with combinations of thesingle metal reflective layer and the pad layer with differentthicknesses according to the present invention.

DETAILED DESCRIPTION

In order to make the structure and characteristics as well as theeffectiveness of the present invention to be further understood andrecognized, the detailed description of the present invention isprovided as follows along with embodiments and accompanying figures.

Please refer to FIG. 1, a light emitting diode (LED) disclosed in thepresent invention comprises a stacked semiconductor layer structuredisposed on a substrate 10. The stacked semiconductor layer structurecomprises, from bottom to top, a first-type semiconductor layer 20, anactive layer 30, and a second-type semiconductor layer 40. The activelayer 30 is disposed between the first- and the second-typesemiconductor layers 20, 40. The second-type semiconductor layer 40 andthe active layer 30 do not cover completely the first-type semiconductorlayer 20. Instead, a portion of the first-type semiconductor layer 20 isexposed for disposing an electrode. The first- and second-typesemiconductor layers 20, 40 can be N-type or P-type galliumnitride-based semiconductors. The active layer 30 can be a multiplequantum well layer. Bias voltages can be supplied via electrodes to makethe active layer 30 emit light. Nonetheless, the present invention doesnot limit the types of these semiconductor materials.

According to the present invention, a first electrode 21 and a secondelectrode 41 are disposed on the same side of the stacked semiconductorlayer structure of the LED. The first electrode 21 is disposed on thefirst-type semiconductor layer 20; the second electrode 41 is disposedon the second-type semiconductor layer 40. The first electrode 21includes a single metal reflective layer 212 and a pad layer 213, wherethe pad layer 213 is disposed on the single metal reflective layer 212.The material of the single metal reflective layer 212 includes aluminumor aluminum alloy with a thickness not less than 1 um. The single metalreflective layer 212 is not the aluminum thin film having a negligiblethickness applied in the LED according to the prior art. Particularly,the thickness of the single metal reflective layer 121 according to thepresent invention is greater than that of the active layer 30.Accordingly, the structure is beneficial to the enhancement oflight-emitting efficacy of LEDs. The detail is described in thedescription regarding the experimental results show in FIGS. 5A, 5B, and6A to 6D below.

According to a preferred embodiment as disclosed in FIG. 1, the heightof the bottom 2121 of the single metal reflective layer 212 is lowerthan the height of the bottom 301 of the active layer 30, and the heightof the top 2122 of the single metal reflective layer 212 is higher thanthe height of the top 302 of the active layer 30. Consequently, at leasta portion of the light emitted from the side surface of the active layer30 will be reflected by the single metal reflective layer 212, as shownin FIG. 4. In other words, by increasing thickness of the single metalreflective layer 212 of the first electrode 21, the side surface of thesingle metal reflective layer 212 can be used as a reflective surface2120. Further, the single metal reflective layer 212 is located near toone side of the active layer 30 and separated by a gap D, it can reflecta portion of the light emitted from the active layer 30, which means thepath of light reflection is increased and the light-emitting efficiencyof the LED can be improved. Wherein the gap D described above is rangedbetween 7-8 um.

The material of the first electrode 21 may include one selected from thegroup consisting of chrome, aluminum, platinum, gold, titanium,tantalum, ruthenium, rhodium, silver, nickel, lead, copper, and an alloythereof The material of the single metal reflective layer 212 mayinclude one selected from the group consisting of aluminum, platinum,titanium, tantalum, ruthenium, rhodium, silver, nickel, lead, and analloy thereof So that the single metal reflective layer 212 may beformed as a thick platinum layer, a thick titanium layer, a thicktantalum layer, a thick ruthenium layer, a thick rhodium layer, a thicksilver layer, a thick nickel layer, a thick lead layer or etc. Accordingto a preferred embodiment, the material of the single metal reflectivelayer 212 may be aluminum or aluminum alloy, and is thickened to form areflective surface on the side of the structure. Hence, it is notrequired to manufacture an additional reflective layer on the side orbottom of the first electrode 21.

Furthermore, as shown in FIGS. 5A, 5B, and 6A to 6D, given the conditionof not altering the composition of the other materials in the firstelectrode 21, although using the single metal reflective layer 212 canimprove the luminous intensity of the LED according to the presentinvention, the electrical properties of the LED are affected. Therefore,according to a preferred embodiment of the present invention, the firstelectrode 21 further includes the gold formed pad layer 213 having athickness not less than 0.8 um when the thickness of the aluminum singlemetal reflective layer 212 is not less than 1 um. For example, when thethickness of the single metal reflective layer 212 is 1.5 um and thethickness of the pad layer 213 is 1 um, in addition to improve thelight-emitting efficiency, the electrical properties of the LEDaccording to the embodiment of the present invention are the same as orclose to that of the regular LED. Besides, in order to reduce themanufacturing process, the first and second electrodes 21, 41 arepreferably fabricated in the identical process. Which means the firstand second electrodes 21, 41 of the LED according to the embodiment willhave the same material and structure while the performance will besimilar or the same as described above. In addition, in order to makethe side surface of the single metal reflective layer 212 be utilized inreflection effectively, the pad layer 213 does not cover the sidesurface of the single metal reflective layer 212. According to apreferred embodiment, the area of the pad layer 213 is the same as thatof the single metal reflective layer 212.

Please refer to FIGS. 2 and 3. An LED according to the present inventionmay further comprise a transparent conductive layer 50 disposed betweenthe second electrode 41 and the second-type semiconductor layer 40.According to another preferred embodiment, the height of the top 2122 ofthe single metal reflective layer 212 is not lower than that of thetransparent conductive layer 50. In addition, a reflective layer 60 isdisposed under the substrate 10 of the LED for reflecting the downwardlight emitted from the active layer 30 upwards and enhancing thebrightness of the LED. Moreover, a side surface or the side reflectivesurface 2120 of the first electrode 21 is parallel with a side surfaceformed by the active layer 30, the second-type semiconductor layer 40and the transparent conductive layer 50, and a gap D exists between thesurfaces. A side surface of the second electrode 41 corresponding to thefirst electrode 21 is parallel with the side surface or the sidereflective surface 2120 of the first electrode 21. Accordingly, inaddition to emitting more uniform light, the distribution of currents inthe LED according to the present invention is more even, which improvesthe light-emitting efficiency of the LED according to the presentinvention.

FIG. 4 shows a schematic diagram in which the light emitted from theactive layer 30 is reflected by the side reflective surface 2120 of thesingle metal reflective layer 212 of the first electrode 21. The bottomof the single metal reflective layer 212 can provide reflection functionas well. Thereby, it is not required to fabricate an additionalreflective layer between the electrodes and the semiconductor layers byusing the electrode structure disclosed in the present invention.

FIGS. 5A and 5B show the comparison in electrical property and luminousintensity of four sets of experimental results between the regular LEDs(STD) and the LEDs having a single metal reflective layer with differentthicknesses according to the present invention. The thicknesses of thepad layers of the regular LEDs and the LEDs according to the presentinvention are 1.8 um and formed by gold. The thicknesses of the singlemetal reflective layers of the LEDs according to the present inventionmade by aluminum are 0.5, 1.0, 1.5, and 2.0 um respectively in the 1'stto 4'th set. As shown in the figures, except the 1'st set that thedifference of experimental results is unobvious, the forward biasvoltage (V.sub.F) of the LEDs according to the present invention islower than that of the regular LEDs by more than 10 mV in the otherthree sets, while the luminous intensity (LOP) is improved by 0.01 to0.04 mcd. According to the experimental results, the luminous intensityof the LED according to the present invention is improved byapproximately 1-2%.

FIG. 6A-6D show comparisons in electrical property of experimentalresults between the regular LEDs (STD) and the LEDs with combinations ofthe single metal reflective layer and the pad layer with differentthicknesses and sampling ratios according to the present invention. Thesampling ratios in the figures are normalized. As shown in the FIG. 6D,the electrical property of the LEDs according to the present inventionis very close to that of the regular LEDs, wherein the first electrode21 comprises a 1.5-um-thick aluminum single metal reflective layer 212and a 1-um-thick gold pad layer 213. Which means, under the condition ofsaid combination of the single metal reflective layer and the pad layer,the electrical property of the LED according to the present invention isthe same as that of the regular LED while providing extra light-emittingefficiency.

To sum up, the present invention discloses an LED comprising a singlemetal reflective layer having a thick aluminum film in its electrodestructure, so that a portion of the light emitted from the active layercan be reflected by the side surface of the single reflected metallayer. By increasing the path of light reflection, the light-emittingefficiency can be enhanced. Besides, the electrical performance of theLED according to the present invention is maintained by the combinationof the pad layer and the single metal reflective layer in particularratio.

However, the foregoing description is only embodiments of the presentinvention, not used to limit the scope and range of the presentinvention. Those equivalent changes or modifications made according tothe shape, structure, feature, or spirit described in the claims of thepresent invention are included in the appended claims of the presentinvention.

1. A light-emitting diode, comprising: a first-type semiconductor layer;an active layer over the first-type semiconductor layer; a second-typesemiconductor layer over the active layer; a first electrode next to asidewall of the active layer, wherein the first electrode comprises asingle metal reflective layer and a pad layer disposed over the singlemetal reflective layer; and a reflective layer under the first-typesemiconductor layer; wherein the height of the bottom of the singlemetal reflective layer is lower than the height of the bottom of theactive layer, the height of the top of the single metal reflective layeris higher than the height of the top of the active layer.
 2. Thelight-emitting diode of claim 1, further comprising a second electrodeover the second-type semiconductor layer.
 3. The light-emitting diode ofclaim 2, further comprising a transparent conductive layer disposedbetween the second-type semiconductor layer and the second electrode. 4.The light-emitting diode of claim 1, wherein a gap with a width between7 um and 8 um is formed between the first electrode and the activelayer.
 5. The light-emitting diode of claim 1, wherein the material ofthe single metal reflective layer includes aluminum or aluminum alloy.6. The light-emitting diode of claim 1, wherein the material of thesingle metal reflective layer includes one selected from the groupconsisting of platinum, titanium, tantalum, ruthenium, rhodium, silver,nickel, and lead.
 7. The light-emitting diode of claim 1, wherein thethickness of the single metal reflective layer is not less than 1 um. 8.The light-emitting diode of claim 7, wherein the thickness of the singlemetal reflective layer is from 1 um to 2 um.
 9. The light-emitting diodeof claim 1, wherein the material of the pad layer includes gold.
 10. Thelight-emitting diode of claim 1, wherein the thickness of the pad layeris not less than 0.8 um.
 11. The light-emitting diode of claim 1,wherein the single metal reflective layer directly connects to the padlayer.
 12. The light-emitting diode of claim 1, wherein the area of thesingle metal reflective layer is substantially the same as that of thepad layer.
 13. The light-emitting diode of claim 1, wherein the padlayer does not cover a side surface of the single metal reflectivelayer.
 14. The light-emitting diode of claim 1, wherein a portion of theupper surface of the first-type semiconductor layer is exposed and thefirst electrode is disposed on the exposed upper surface.
 15. Thelight-emitting diode of claim 1, further comprising a substrateinterposed between the reflective layer and the first-type semiconductorlayer.
 16. The light-emitting diode of claim 1, wherein at least aportion of light emitted from the sidewall of the active layer isreflected by the single metal reflective layer.